Officials in Tokyo would not confirm Wednesday the reports of a potential shift in Japan's policy, but government spokesman Yoshihide Suga also declined to explicitly deny a summit was being explored.

"What is important is that the three countries, Japan, the US and South Korea, continue to closely coordinate policies," he said at a regular briefing.

"We wish to make progress on our efforts to reach a comprehensive resolution of the nuclear, missile, and abduction (of Japanese citizens)... and as part of that, we will review our way forward from the perspective of the most effective approach."

There has been no public suggestion from any side of an Abe-Kim summit.

But Japan's Kyodo and Jiji Press agencies both reported the possibility was being studied after Tokyo found itself on the back foot with the surprise announcement last week that US President Donald Trump would meet with Kim later this year.

Japan has long maintained a hardline position on negotiations with Pyongyang, warning against "talks for the sake of talks", and in the early days of Trump's administration there appeared to be little daylight between Tokyo and Washington on the issue.

But the prospect of a Trump-Kim summit, as well as talks between North and South Korea, appears to have pushed Tokyo to reassess its position, analysts said.

Japan fears the current diplomatic track could leave unaddressed the North's short-range missile capabilities, as well as the emotive issue of Japanese citizens kidnapped by Pyongyang during the Cold War.

"Japan has to hold a summit as soon as possible after the North-South summit and the US-North summit... to avoid being left out both in terms of security concerns and on the abduction issue," Masao Okonogi, a North Korea specialist affiliated with Tokyo's Keio University, told AFP.

Speaking before the latest reports, Korea expert Kan Kimura, a professor at Kobe University, told AFP that Japan had been "left behind" by the recent diplomatic activity.

"Things are moving forward without Japan," he said.

Fear of isolation

The Japanese are "showing clear signs that they are deeply unsettled by this," added Daniel Sneider, an Asia-Pacific specialist at Stanford University.

"The Japanese fear being isolated because they have basically positioned themselves as the most hardline of hardliners, constantly expressing scepticism about the North Korean openings to dialogue," he added.

The last Japanese prime minister to meet a North Korean leader was Junichiro Koizumi, who met Kim's father Kim Jong Il in Pyongyang in 2004.

But Abe rose to political prominence on his calls for a tough line on North Korea, and attitudes only hardened after two missiles fired by Pyongyang flew over Japan last year.

The hostility goes both ways, with Pyongyang regularly excoriating Japan, including over its colonial rule of the Korean peninsula in the early 20th century.

"Korea is not a weak nation as it used to be in the past when it was trampled under the jackboots of the Japanese imperialists," the official Korean Central News Agency said in a commentary Monday.

The prospect of an Abe-Kim summit also comes at a time of political turmoil for Japan's premier, who is under fire at home over the discount sale of government land to one of his supporters, a deal that was the apparent subject of a cover-up by the finance ministry.